Allow to define modules whose name appears on Object.prototype (#196)

One of the CommonJS tests adopted in #195 exposed a defect in Cordova's module system:

When trying to define a module with a name that also is a property of Object.prototype, an exception would occur.

src/scripts/require.js

@@ -67,7 +67,7 @@ var define;
  };
 
  define = function (id, factory) {
- if (modules[id]) {
+ if (Object.prototype.hasOwnProperty.call(modules, id)) {
  throw 'module ' + id + ' already defined';
  }
 

test/test.require.js

@@ -231,7 +231,7 @@ describe('require + define', function () {
  });
 
  // Adapted version of CommonJS test `hasOwnProperty`
- xit('Test#018 : allows properties of Object.prototype as module names', () => {
+ it('Test#018 : allows properties of Object.prototype as module names', () => {
  expect(() => {
  define('hasOwnProperty', jasmine.createSpy());
  }).not.toThrow();